Conventionally, for example, Patent Literature 1 teaches a semiconductor device in which a concavity is formed in a longitudinal direction anisotropic conductive sheet, a latitudinal direction anisotropic conductive sheet is arranged in the concavity so that a heat spreader is formed, and a semiconductor element is mounted on the latitudinal direction anisotropic conductive sheet in the heat spreader. Here, the longitudinal direction anisotropic conductive sheet is a sheet having a thermal conductivity in the longitudinal direction higher than the thermal conductivity in the latitudinal direction. The latitudinal direction anisotropic conductive sheet is a sheet having a thermal conductivity in the latitudinal direction higher than the thermal conductivity in the longitudinal direction.
When heat is generated at the semiconductor element in the above semiconductor device, the heat is transferred to the latitudinal direction anisotropic conductive sheet in the heat spreader at first. Then, when the heat is conducted to the latitudinal direction anisotropic conductive sheet along a latitudinal direction (i.e., a planar direction of the heat spreader), the heat is transferred to the longitudinal direction anisotropic conductive sheet arranged on a side of the concavity. After that, the heat is conducted to the longitudinal direction anisotropic conductive sheet along a longitudinal direction (i.e., a thickness direction of the heat spreader), and the heat is irradiated to an outside.
However, in the above semiconductor device, when the heat generated at the semiconductor element is transferred to the latitudinal direction anisotropic conductive sheet, the heat is conducted along the latitudinal direction of the latitudinal direction anisotropic conductive sheet. However, it is difficult to conduct the heat along the longitudinal direction of the latitudinal direction anisotropic conductive sheet. Accordingly, the above semiconductor device actually has a difficulty such that heat radiation may be reduced at a portion below the semiconductor element, i.e., a portion disposed on a bottom side of the concavity in the longitudinal direction anisotropic conductive sheet. Thus, there is a difficulty such that the heat radiation at the portion of the heat spreader opposite to the semiconductor element may be lower than the heat radiation at another portion around the portion, which is disposed below the bottom of the concavity, so that the heat generated at the semiconductor element is not effectively radiated.